Extending setindex for vector indexing

Learning from my experiments on #745 i looked back at the missing support for vector indexing in setindex.

@inline function setindex(a::SVector{L}, v::AbstractArray, indices::AbstractArray) where L
    out = MVector(a)
    for (i, x) in zip(indices, v)
        # Do boundscheck here to allow Julia to fully elide the dynamic allocation.
        @boundscheck if (i < 1 || i > L)
            throw(BoundsError(a, i))
        end
        @inbounds out[i] = x
    end
    return SVector(out)
end

while we're at it, looks like setindex for scalars can easily be written this way as well.

@inline function setindex2(a::SVector{L}, x, index::Integer) where L
    out = MVector(a)
    # Do boundscheck here to allow Julia to fully elide the dynamic allocation.
    @boundscheck if (index < 1 || index > L)
        throw(BoundsError(a, index))
    end
    @inbounds out[index] = x
    return SVector(out)
end

Looking at assembly, it looks nicer for this implementation. I have had some mixed results trying to benchmark this; peaking at @code_native looked very efficient, but benchmark times still fell a bit short. Perhaps my benchmarks were lacking, or i missed something.

So is this interesting?

Start by locating the existing setindex methods for SVector and review the related discussion in issue #745. Compare vector and scalar indexing behavior, including the shown bounds checks, then use assembly inspection and benchmarks to verify the intended implementation. Done means both forms work with correct bounds errors and their performance is validated.